Fraunhofer line discriminators are systems for sensing, measuring and recording fluorescence reflected from certain materials in a scene viewed in sunlight. Such instruments became feasible with the discovery that sunlight reflected from the earth contains the identical Fraunhofer absorption lines which exist in direct sunlight. Fraunhofer lines which are unique to sunlight produce fingerprint information that characterizes sunlight. In contrast fluroescence produced by reflected sunlight does not have the Fraunhofer absorption lines. Thus, fluorescence caused by reflected sunlight can be sensed and measured by accurate comparison of light level within a selected Fraunhofer absorption line to the level in an adjacent spectral region in which there is no Fraunhofer absorption line. In its simplest form an apparatus for measuring fluorescence produced within a sunlit area of earth employs a narrow band filter tuned to pass a selected Fraunhofer absorption line, means to detect intensity of reflected light within the selected Fraunhofer line and means to detect intensity of reflected light just outside the selected Fraunhofer. Solution of a simple algebraic expression involving these two quantities provides the intensity of fluorescence of any given point on earth.
Various techniques have been developed to measure fluorescence within a sunlit scene. Some of these techniques are disclosed in U.S. Pat. Nos. 3,598,994; 3,641,344; and 3,769,516 all having the same assignee as the present invention.
While the above patents do not directly address it, a problem associated with present methods of Fraunhofer line discrimination is the narrow field of view through which the necessary discrimination data must be obtained. In many cases the field of view 1.degree. or less. This severely limits the width of the scene viewable by Fraunhofer line discriminators.
The present invention permits the detection of fluorescence over a much wider field of view from high altitudes with good spatial resolution without sacrifice of the narrow spectral interval imposed by the Fraunhofer absorption lines.